Repairing driver's seat position adjustment

Here's one from long ago. I remember I'd hop into the car, and sometimes when I would accelerate quickly (ok, as quickly as a 40-year old car can), my seat would suddenly jump back to the furthest position on the rails! Not great for me, as it would make the clutch and brake a stretch to reach. So I removed the seat, and here's what I found:

Teeth have worn through the square slots for adjustment

I don't have a clue how the original owner could have worn right through the square slots... maybe the kids would get in and slide the seat back and forth? Anyhow, I removed the adjustment lever, then took it to a local welding shop and had them fill the spaces in again.